Should I repeat the 'Umrah that I previously performed based on a fatwa that it was Sunnah, then later learned of its obligation?

Your performance of Umrah (definition) as a Sunnah (definition) is valid and sufficient for the obligatory Umrah of Islam, and you are not required to repeat it, because it was performed in a valid and religiously recognized manner. The Malikis and most of the Hanafis hold that Umrah is a confirmed Sunnah, while the Shafi'is and Hanbalis consider it an obligation (fard). The intention of obligation is not a condition for Umrah; the intention of performing the act itself is sufficient, as is the case with Hajj and purification (tahara).